The energy equivalents given in the literature are reviewed and criticized. New equivalents are calculated for protein respiration, The energy equivalent for converting rate of O2 consumption into rate of heat production (Qox cal mg-1 O2 consumed) is 3.53 cal mg-1 for carbohydrate oxidation, 3.28 cal mg-1 (range 3.22-3.32) for fat oxidation. Qox values for the respiration of standard protein are the same at 3.25 cal mg-1 for urotelic and uricotelic animals, and {approx} 2% less at 3.20 cal mg-1 for ammoniotelic animals. The energy equivalent for converting rate of O2 consumption into rate of energy loss in excreta (Qex cal mg-1) varies considerably with different excretory products. Values for standard protein are 0.62 cal mg-1 for ammonioteles. 0.58 cal mg-1 for ureoteles, and 0.94 cal mg-1 for uricoteles. The various factors affecting both Qox and Qex are discussed, and examples of the estimation of general energy equivalents are given.